The resolution of the bootstrap program was systematised over the years and these efforts led to explicit expressions for the form factors of local operators in
numerous 1+1 dimensional massive quantum field theories, see e.g.
[1]}. The first expressions for the form factors were rather combinatorial in nature. Later, a substantial progress was achieved in simplifying the latter,
in particular by exhibiting a deeper structure at their root. Notably, one can mention the free field based approach, also called angular quantisation,
to the calculation of form factors. It was introduced by Lukyanov [2]} and allowed to obtain convenient representations for certain form factors solving the bootstrap program.
In particular, the construction lead to closed and manageable expressions [3]}, [4]} for the form factors of the exponential of the field operators in the Sinh-Gordon and the
Bullough-Dodd models. Later, Babujian, Fring, Karowski, Zapetal [5]} and Babujian, Karowski [6]}, [7]}
developed the more powerful \(\mathcal {K}\) -transform approach which will be described below on the example of the Sinh-Gordon model.
The construction of [6]}, [7]} was improved in [10]}, [11]}
so as to encompass more complicated operators, the so-called descendants of the Sinh-Gordon exponential of the field operator.
